The present invention relates to an optical unit which modulates the intensity of a light beam from an illumination optical system by the polarization function of a reflection image display device to make an optical image and projects the image on a screen through a projection optical system, and a projection type image display apparatus which uses the unit.
A popularly known projection type image display apparatus is a projector which has an optical unit housed in a case together with power supply circuitry and image drive circuitry where the optical unit makes an optical image by light intensity modulation and projects the image on a screen through a projection lens. Here, light intensity modulation refers to a process where light from a light source is polarized to change the shading of each picture element according to image signal through a light valve device.
When a reflection liquid crystal panel is used as a light valve device, usually a polarizing beam splitter prism (hereinafter called a PBS prism) is used as a polarizer/analyzer. A relatively inexpensive PBS prism has a dielectric multilayer film which transmits p-polarized light and reflects s-polarized light (the multilayer film is hereinafter called the PBS film).
When a PBS prism is used as a polarizer/analyzer, a reflection type liquid crystal projector requires a quarter-wave plate for reducing light leakage from the PBS prism for black image in order to increase the contrast. However, even when the quarter-wave plate is used, its effect is not satisfactory.
Generally, a quarter-wave plate has wavelength and angle characteristics. Therefore, as the difference of the incident light wavelength from the design center wavelength increases or the incidence angle increases, the quarter-wave plate performance decreases. In a reflection type liquid crystal projector in which the light impinging on the reflection liquid crystal panel has a certain wavelength range and a certain angle range, the effect of reducing leakage of all incident light is not perfect.
A conventional method of preventing this leak light from being projected on the screen is to place a polarizing plate between the PBS prism and the projection lens. However, it is impossible to prevent light leakage completely because the leak light includes rays polarized in the same direction as the direction of the polarizing plate transmission axis.
Besides, the use of the PBS prism is disadvantageous from the viewpoint of weight reduction. Furthermore, the PBS prism must use a glass material whose photoelastic coefficient is low because it is necessary to avoid deterioration in contrast which might be caused by light leakage due to polarized light turbulence in transmission of rays through the glass. However, generally such a glass material has a large specific gravity and is expensive because it is not widely available on the market.
In order to solve the above problems, a reflection type polarizing plate may be used as a polarizer/analyzer, as described in U.S. Pat. No. 6,234,634 or catalog No. PBF02A of Moxtek (US). This type of reflection polarizing plate functions as a polarizing plate on a condition that the optical diffraction grating reflects polarized light rays parallel to the grating and transmits polarized light rays perpendicular to it.
Therefore, this structure eliminates the possibility of oblique leak light, which might occur in a PBS prism-based structure, and thus theoretically provides higher contrast than the PBS prism-based structure.